[A-CG50] Biogeochemical linkages between the surface ocean and atmosphere

[ACG50-09] Vertical profile of volatile organic iodine compounds in seawater of the subarctic, subtropical and tropical North Pacific

*Atsushi Ooki¹, Tianchang Cui¹, Toshikazu Tatematsu¹, Yugo Kanaya², Fumikaze Taketani², Takashi Sekiya², Hisahiro Takashima³, Kazuhiko Takeda⁴, Yoko Iwamoto⁴, Toru Hirawake⁵, Jun Nishioka¹, Hiromichi Ueno¹ (1.Hokkaido University, 2.JAMSTEC, 3.Fukuoka University, 4.Hiroshima University, 5.National Institute of Polar Research)

Keywords:Iodine cycle, Volatile Organic Compounds (VOCs), Photolysis

Shipboard observations were conducted in the North Pacific from subarctic to tropical in May, June, or July of 2016 and 2023, to clear the typical feature of vertical profiles of volatile organic iodine compounds (VOIs) in water column. In the oligotrophic tropical and subtropical waters, where the euphotic layer was strongly stratified, concentrations of CH₂ClI and CH₂I₂ showed minimum in the surface mixed layer (0 – 14 dbar) and conspicuous maximum near the chlorophyll-a maximum depth (74 – 123 dbar). The feature of CH₂ClI profile was similar to the CH₂I₂, but, the CH₂ClI maximum depth was approximately 25 dbar shallower than the CH₂I₂ maximum. In the subarctic water, broad peaks of CH₃I, CH₂ClI, and CH₂I₂ profiles were found in the euphotic layer approximately 0 – 40 dbar. In the surface mixed layer (ML) of all study areas, CH₃I dominated among the 3 VOIs accounting for 55 – 95% of total VOIs concentration. The integrated amounts of total VOIs in the depth range of 0 – 300 m in the tropical water (3.25 – 4.16 μmol/m²) was much higher than the subarctic (0.61 – 1.33 μmol/m²). From the UV-A attenuation rate analysis, we considered that CH₂ClI production in the surface mixed layer of subarctic water was much lower than the tropical and subtropical mixed layers. We found that VOIs remained and accumulated in the subsurface waters for long period of time (0.5 – 2.5 years). We concluded that the vertical profiles of VOIs are controlled by present and past biological productions, photolysis by UV-A, vertical mixing, stratification, and surface and subsurface water advections.